Epoxides, process for their preparation and their use for the preparation of perfuming ingredients

ABSTRACT

Epoxides of formula                    
     having a cyclanic (1R) configuration, the group in position 2 being in a trans configuration and the epoxy group being in a cis configuration with respect to that in position 1 of the ring, and in which R represents a lower alkyl group and R 1  represents a saturated or unsaturated, linear or branched hydrocarbon group having from 1 to 8 carbon atoms, are useful molecules for the preparation of very prized perfuming ingredients. A process for the preparation of the epoxides (I) is also described.

This application is a division of 08/960,203,filed Oct. 29, 1997, now U.S. Pat. No. 5,962,706.

BRIEF SUMMARY OF THE INVENTION

The present invention concerns the field of organic synthesis. More particularly, it concerns epoxides which are useful as starting products for the preparation of highly prized perfuming ingredients, amongst which the isomers of Hedione® (methyl-3-oxo-2-pentyl-1-cyclopentaneacetate; origin: Firmenich SA, Geneva, Switzerland) which are preferred from an olfactive point of view.

Thus, an object of the present invention is an epoxide of formula

having a cyclanic (1 R) configuration, the group in position 2 being in a trans configuration and the epoxy group being in a cis configuration with respect to that in position 1 of the ring, and in which R represents a lower alkyl group and R¹ represents a saturated or unsaturated, linear or branched hydrocarbon group having from 1 to 8 carbon atoms.

By lower alkyl group it is meant here a linear or branched alkyl group from C₁ to C₄, and more particularly a methyl or ethyl group.

The structure of the epoxides of formula (I) is novel.

PRIOR ART

International patent application WO 95/33735, published on Dec. 14, 1995, describes epoxides of formula

in which R³ stands for an alkyl group having from 1 to 8 carbon atoms and R^(b) for an alkyl group from C₁ to C₄. However, one cannot detect therefrom a description or any mention of the precise stereochemistry of compounds (A). Now, trying to reproduce the process described in this document for the preparation of these compounds, we have discovered that it only allows the preparation of a mixture of diastereomers of each epoxide of formula (A), namely in the cases in which R^(a) is a pentyl or hexyl group and R^(b) a methyl or ethyl group. We have thus found that the process for the preparation of epoxides (A) as described in WO 95/33735 is not diastereoselective and that, moreover, the reaction occurs with very low conversion, of the order of 25%.

We have also discovered that, contrary to what is indicated in WO 95/33735, the transformation of the thus-obtained epoxide, under the described reaction conditions, namely the thermal treatment of the epoxide in presence of a catalytic amount of lithium iodide, leads to the formation of a product which is totally distinct from the desired ketone. So, when applying the process as described in WO 95/33735 to the epoxide of formula (A) wherein R^(a)=pentyl and R^(b)=CH₃, we obtained, instead of methyl-3-oxo-2-pentyl-1-cyclopentaneacetate as expected according to the description in the said document, a totally different lactone which was identified by its spectral data as having the structure

i.e. cis-perhydro-6a-pentyl-2-cyclopenta[b]furanone.

It is in fact a lactone which has been known in our laboratories for a long time, possessing a watery-metallic, chemical odor, which is slightly animal. However, not even a trace of the desired cyclic ketone could be detected in this product. The same applied to the cases where R^(a)=hexyl and R^(b)=methyl or ethyl.

In summary, is apparent from the above that the above-mentioned prior art document does not describe a process which allows either to obtain or to convert the epoxides of formula (1) according to the present invention and that as a result there was no recognition of any particular usefulness of the compounds which are the object of the present invention and have the specific stereochemistry indicated above.

Yet, we have now discovered that compounds (I) are very useful as starting products for the preparation, in a pure state, of optically active perfuming ingredients which are particularly prized by the perfumers.

DETAILED DESCRIPTION OF THE INVENTION

It should also be noticed that, as will be apparent from the comparative examples presented further on, the above-mentioned prior art document does not describe the pure racemates corresponding to epoxides (I), i.e. the epoxides corresponding to the formula

in which the epoxy and carboxy substituents have a relative configuration which is strictly cis and the symbols R et R¹ have the meaning given in formula (I). These compounds, which are equimolar mixtures of an epoxide (I) with its respective enantiomer, are also novel compounds. They turn out to be useful for the preparation of the corresponding ketones having a strictly cis cyclanic configuration and the most interesting example of which, in perfumery, is cis-Hedione® or cis-methyl-3-oxo-2-pentyl-1-cyclopentaneacetate.

According to the invention, the optically active epoxides of formula (I) are preferred compounds and one can cite, as an even more preferred compound, (+)-methyl (1 R,2 S,3 R)-2,3-epoxy-2-pentyl-1-cyclopentaneacetate. The transformation of this compound according to the original process described hereinafter allows the preparation of (+)-cis-Hedione®, or (+)-methyl (1 R)-cis-3-oxo-2-pentyl-1-cyclopentaneacetate, i.e. the isomer which is known to possess at best the typical odor characteristics, in particular the jasmine note, of this well-known cyclic ketone.

Therefore, the invention also concerns the use of epoxides of formula (I) for the preparation of cyclic ketones according to the formula

having a (1 R)-cis cyclanic configuration, according to an original conversion process, comprising the treatment of said epoxide in an inert organic solvent with an acidic agent consisting of an appropriate Lewis acid or an acidic clay.

We have discovered with surprise that these reaction conditions made it possible to obtain the optically active ketones of formula (II) in a selective way and excellent yields, as will be apparent from the examples below. This result is contrary to what could be observed with the prior art process as described in the above-cited document, according to which the epoxide was subjected to a thermal treatment in the presence of a salt of an alkaline or alkaline earth metal of an acid of low molecular weight.

The reaction which characterizes the process of the invention is executed at various temperatures which depend on the reagents used and in particular on the reactivity of the acidic agent. Therefore, and contrary to the process described in WO 95/33735, the process of the invention allows the use of mild temperature conditions, typically below 120° C.

As Lewis acids which are appropriate for the process of the invention, one can cite in particular boron trifluoride, which may be complexated (as etherate, for example), aluminum trichloride and magnesium iodide.

As acidic clays, one can use aluminosilicate based clays from the family of products known under the name of FILTROL® (origin: Harshaw/Filtrol), clays which are commercially available under designations like GK (origin: Georgia Kaolin Co.), montmorillonites known under the name of K catalysts, for example K10, KP10, KSF et KSF/O (origin: S{umlaut over (u)}id-Chemie AG), bentonites, or yet any other acidic clay of current use. According to a preferred embodiment of the invention, one will use, for example, Filtrol® G24.

The proportion in which the acidic agent can be used relative to the starting epoxide varies within a large range of concentrations. We have observed that the ratio between the two reagents could be stoichiometric for example, but equally useful results were obtained with catalytic amounts of acidic agent relative to the epoxide.

As solvents which can be used according to the invention, one can employ a solvent of current use which is inert under the reaction conditions. One can cite in particular cyclic or acyclic hydrocarbons, in particular cyclohexane, xylene and toluene, ethers like diethyl ether or tetrahydrofurane, or even chlorinated solvents, in particular dichloromethane. Particularly useful results were obtained with toluene or dichloromethane.

According to a preferred embodiment of the process of the invention, (+)-methyl (1 R,2 S,3 R)-2,3-epoxy-2-pentyl-1-cyclopentaneacetate is used. This embodiment revealed itself particularly advantageous, since, as is cited above, it allows the preparation of the preferred isomer of Hedione®, namely the (+)-methyl (1 R)-cis-3-oxo-2-pentyl- 1-cyclopentaneacetate.

Of course, when using in the above-described process a racemic epoxide of formula (I′) as defined above, the corresponding racemic cyclic ketones will be obtained, which are the racemic equivalents, having a strictly cis cyclanic configuration, of the ketones of formula (II). Thus, when using for example methyl c-2,3-epoxy-2-pentyl-r-1-cyclopentaneacetate, racemic cis-Hedione® will be obtained in a pure state.

The invention also concerns a process for the preparation of epoxides of formula (I) previously defined, characterized in that an ester of formula

having a cyclanic (1 R) configuration and wherein symbols R and R¹ have the meaning indicated in formula (I), is reacted with a strong peracid in an inert organic solvent.

The optically active esters of formula (III) are novel compounds, prepared according to an original process which will be described further on.

In fact, even though the racemic methyl 2-pentyl-2-cyclopentene-1-acetate is known from the article of T. Kitahara et al., in Agric. Biol. Chem. 50, 1867 (1986), there is no mention in this article of one or the other of the enantiomers which are present in this racemic compound, namely that of (1 R) configuration which obeys formula (III). Now, as will be seen further on, the present invention provides an original process for the preparation of the desired enantiomers of formula (III) having the absolute configuration (1 R), and more particularly of (+)-methyl (1 R)-2-pentyl-2-cyclopentene-1-acetate.

As the strong peracid used according to the invention in the preparation of epoxides of formula (I), one can use organic peracids which are strongly electron-attracting, chosen amongst the peroxyacids of current use in oxidation reactions (see, for example, chapter III in Org. Chem., 5, ed. W. S. Trahanovsky, Academic Press, 1978). In this context, permaleic acid, perphthalic acid, m-chloroperbenzoic acid or yet the peracids of formula CX₃COO₂H, wherein X typically stands for a halogen atom, in particular chlorine or fluorine, can be cited as preferred agents. Preferably, permaleic or trifluoroperacetic acid will be used.

As appropriate solvents in this process, a chlorinated solvent will be preferably used. Very useful results have been obtained in particular with dichloromethane.

The esters of formula (III) used as starting products in the process described above are novel compounds, which are also an object of the invention. Amongst these esters, one can cite in particular (+)-methyl (1 R)-2-pentyl-2-cyclopentene-1-acetate, the conversion of which, according to the present invention, allows the preparation of the preferred isomer of Hedione®.

The invention also concerns an original process for the preparation of the esters of formula (III), starting from commercially available materials or products which can be prepared using current reactions. The reactions which characterize the preparation process of esters (III) are schematically represented hereinafter:

According to the invention, the first reaction in this scheme consists of an enantioselective reduction of the starting cyclopentenone which is carried out by means of a system of the oxazaborolidine-borane type, which, with respect to the substrate, can be used in stoichiometric or catalytic amounts. This is a system for the enantioselective reduction of ketones which is described in various publications (see, for example, E. J. Corey et al., J. Amer. Chem. Soc. 1987, 109, 7925; S. Itsuno et al., Bull. Chem. Soc. Jpn, 1987, 60, 395 ; D. J. Mathre et al., J. Org. Chem. 1991, 56, 751; V. K. Singh, Synthesis 1992, 605), but which, to our knowledge, has never been used in the reduction of cyclopentenones such as cited above. It involves the use of the oxazaborolidine of formula

or (S)-tetrahydro-1-methyl-3,3-diphenyl-1 H,3 H-pyrrolo[1,2-c][1,3,2]oxazaborole as catalyst for the reduction, said reduction being effected using BH₃ (in complexed form). The reduction reaction delivers, further to the desired alcohol (IV), (S)-α,α-diphenyl-2-pyrrolidinemethanol, which can be isolated and re-converted into the oxazaborolidine mentioned beforehand, in a way known per se (see D. J. Mathre, ref. cited).

According to a more advantageous embodiment, the above-mentioned oxazaborolidine is used in catalytic amounts with respect to the cyclopentenone (V), to give the alcohols (IV) having an optical purity of at least 90% e.e. The detailed conditions of these reactions will be described further on.

We were also able to develop a new catalyst for the above-mentioned reduction according to the following formula

in which, in comparison to the known oxazaborolidine catalyst described above, one of the phenyl groups is replaced by a hydrogen atom. In the above formula, the aryl substituent may be an unsubstituted or substituted phenyl group, like toluene, xylene, di-tert-butylbenzene or mesitylene, or even a fused aryl group, like naphthalene.

The new catalyst can be synthesized via a reaction which is analogous to that for the preparation of the known oxazaborolidine catalyst carrying two phenyl groups, which reaction uses, as starting product, the aminoalcohol of the following formula

wherein Ar has the meaning indicated above. The aminoalcohols are prepared according to a known process (see. A. Ookawa, K. Soai, J. Chem. Soc. Perkin Trans. 1, 1987, 1465) and reacted with trimethylboroxine (BOMe)₃ to yield the desired catalyst.

These catalysts are new chemical entities which are also an object of the invention.

When using this new catalytic system with Ar=phenyl, namely (S)-tetrahydro-1-methyl-3-phenyl-1 H,3 H-pyrrolo[1,2,-c][1,3,2]oxazaborol, in the enantioselective reduction according to the present invention, we obtained results which were as good as those obtained with the known oxazaborolidine cited previously. As the latter, the new catalysts can be used in stoichiometric or catalytic amounts with respect to the cyclopentenone (V).

One further synthetic way to enantioselectively reduce the cyclopentenones of formula (V) into alcohols (IV) is a process involving a biotechnological step. Said ketones can be conventionally converted into the corresponding racemic acetates, and the latter then enantioselectively saponified into the desired optically active alcohols (IV) by means of a lipase.

Suitable lipases for this reaction step are known to a person skilled in the art and include for example Candida antarctica, Pseudomonas fluorescens, Pseudomonas Amano, Humicola lang., Candida cylindracea, Mucor Miehei, Chromabacterium viscosum, Aspergillus niger, Mucor javanicus and Rhisopus arrhizus.

The transformation of the keto function into an acetate function can be effected using agents of current use in the art. One can cite in this context the system LiAlH₄/acetic anhydride, but other systems can be used. Likewise, other esters besides the acetate may be used in the process.

The Claisen rearrangement of the allylic alcohols (III) made it possible to obtain the esters of formula (lII) with total retention of the enantiomeric configuration. This is quite a surprising result, given the fact that one would have expected a considerable degree of racemization of the starting cyclopentenol via reversible dehydration, which is favored by the acidic conditions and the elevated temperature of the reaction medium. Yet, we observed practically no loss (less than 1% racemization) in the optical purity of the obtained product, relative to that of the starting cyclopentenol.

The conditions of these esterification reactions are described in further detail below.

In summary, the present invention allows the preparation of new optically active compounds which are themselves useful for the preparation of precious perfuming ingredients and which allow the preparation of the latter in very useful yields and with excellent diastereochemical and enantiomeric purities. And this, by making use of reactions which can be applied on an industrial scale, without danger for the environment.

The invention will now be described in greater detail by means of the following examples, in which the temperatures are indicated in degrees centigrade and the abbreviations have the usual meaning in the art.

EMBODIMENTS OF THE INVENTION EXAMPLE 1

Preparation of (+)-(1 R)-2-pentyl-1-cyclopenten-1-ol

A solution of 2-pentyl-2-cyclopenten-1-one (9.12 g, 60 mmole) in 140 ml of tetrahydrofurane (THF) was treated with an oxazaborolidine represented above, namely (S)-tetrahydro-1-methyl-3,3-diphenyl-1 H,3 H-pyrrolo[1,2-c][1,3,2] oxazaborole (16.7 ml; 0.36 M in toluene, 6.0 mmole; see D. J. Mathre, ref. cited). A solution of BH₃.S(CH₃)₂ (3.30 ml=2.64 g, 34.8 mmole) in THF (60 ml) was added dropwise at 0° over 1 h. The reaction was stopped with 3 ml of methanol (development of H₂), followed by ethyl ether and 5% HCl. The organic phase, which had been washed to neutrality (2x with water and 1x with brine), was separated and the solvent evaporated. 9.5 g of a crude product were obtained, which was distilled in a bulb-to-bulb apparatus (oven temp. 60°/11 Pa), to give 8.1 g of (+)-(1 R)-2-pentyl-2-cyclopenten-1-ol (yield 89%).

The compound showed the following analytical data.

[α]²⁰ _(D)=+28.3(c=2.7; CHCl₃)

enantiomeric excess (e.e.)=91% (measured on a chiral column of type Megadee® 5 or CD-Chirasil® DEX CB on the basis of the corresponding trimethylsilyl ether)

NMR(¹H, 360 MHz): 0.89(t, J=7, 3 H); 1.20-1.40(m, 4 H); 1.40-1.57(m, 3 H; with D₂O=2 H); 1.69(m, 1 H); 2.02-2.47(m, 5 H); 4.64(broad , 1 H); 5.53(broad s, 1 H)δppm.

NMR(¹³C):146.7(s); 126.9(s); 78.9(d); 34.1(t); 31.9(t); 29.7(t); 28.1(t); 27.5(t); 22.6(t); 14.1(q)δppm.

MS:154(M⁺, 2), 97(30), 83(100), 79(12), 67(7), 55(11), 41(9).

When using a different oxazaborolidine, namely (S)-tetrahydro-1-methyl-3-phenyl-1H,3 H-pyrrolo-[1,2-c][1,3,2]oxazaborol (see Example 7), as catalyst in the asymmetric reduction, and in an amount of 10% relative to the substrate, the same chiral cyclopentenol was obtained in a yield of 68% and an e.e. of 90%.

EXAMPLE 2

Preparation of (+)-ethyl (1 R)-2-pentyl-1-cyclopentene-1-acetate

A mixture of the cyclopentenol obtained according to Example 1 (5 g, 32.5 mmol) with triethyl orthoacetate (47.2 ml=42.1 g, 260 mmole, 8 eq.) was heated to 145° (temp. of oil bath 160°) under a nitrogen current in a reactor which was equipped with a Vigreux column and a condenser, while adding continuously, over 4 h, pivalic acid (232 mg, 2.28 mmole, 7 mol %) in triethyl orthoacetate (5.9 ml=5.26 g, 32.5 mmole, 1 eq.). During the reaction, the formed ethanol, as well as the ethyl acetate and a little orthoacetate, were removed by distillation. The reaction mixture was heated for 1 more hour, then cooled to 20°, hydrolyzed with diluted aqueous NaHCO₃ and extracted with ether. The organic layer was washed 2x with water and once with brine, dried over Na₂SO₄ and evaporated, to give 15.4 g of crude product. After distillation of the heads (6.6 g, 50°/1.1×10² Pa), 5.30 g (95°/1.1×10² Pa) of (+)-ethyl (1 R)-2-pentyl-2-cyclopentene-1-acetate were obtained (yield 73%), having an enantiomeric purity of 90% e.e. (chiral column of the type Megadex® 5).

[α]²⁰ _(D)=+23.6(c=3.9; CHCl₃)

NMR(¹H, 360 MHz): 0.89(t, J=7, 3 H); 1.26(t, J=7, 3 H); 1.22-1.63(m, 7 H); 1.86-2.36(m, 6 H); 2.53(dd, J=15 and 4, 1 H); 2.95(broad, 1 H); 4.14(q, J=7, 2 H); 5.37(broad s, 1 H)δppm.

NMR(¹³C): 14.1(q); 14.3(q); 22.6(t); 27.4(t); 28.9(t); 30.4(t); 30.5(t); 31.8(t); 38.8(t); 43.7(d); 60.2(t); 124.3(d); 146.3(s); 173.4(s)δppm.

MS:224(M⁺, 24), 176(5), 150(31), 136(68), 121(20), 107(30), 93(56), 80(100), 79(92), 67(58), 55(18), 41(46), 29(63).

EXAMPLE 3

Preparation of (+)-methyl (1 R)-2-pentyl-2-cyclopentene-1-acetate

The cyclopentenol previously described was reacted in a similar way to that described in Example 2, but using trimethyl orthoacetate (27 eq.) and 10 mole-% of pivalic acid. The reaction was run at a temperature of about 115° over a time of about 6 h. After the usual treatment, the desired (+)-methyl (1 R)-2-pentyl-2-cyclopentene-1-acetate was obtained in 43% yield and with an enantiomeric purity of 90% e.e. (chiral column Megadexo® 5).

The compound showed the following analytical characteristics:

[α]²⁰ _(D)=+25.3 (c=3.6; CHCl₃)

NMR(¹H, 360 MHz): 0.89(t, J=7, 3 H); 1.20-1.60(m, 7 H); 1.85-2.35(m, 6 H); 2.54(dd, J=15 and 4, 1 H), 2.93(broad, 1 H); 3.67(s, 3 H); 5.37(broad s, 1 H)δppm.

NMR(¹³C): 14.1(q); 22.6(t); 27.4(t); 28.9(t); 30.4(t); 30.5(t); 31.8(t); 38.5(t); 43.6(d); 51.5(q); 124.4(d); 146.2(s); 173.8(s)δppm.

MS:210(M⁺, 32), 178(3), 150(16), 136(59), 121(13), 107(23), 93(43), 80(100), 67(42), 41(36), 29(31).

This ester was also prepared, in an identical chemical and optical purity, by trans-esterification of its homologue, described in Example 2 (yield 85%), or saponification of the said homologue to form the corresponding acid, which was then esterified (global yield 85% ; conventional conditions).

The intermediate (+)-(1 R)-2-pentyl 2-cyclopentene-1-acetic acid (yield 89%) showed an enantiomeric purity of 90% e.e. and the following analytical data:

[α]²⁰ _(D)=+32.5 (c=6.0; CHCl₃)

NMR(¹H, 360 MHz): 0.89(t, J=7, 3 H); 1.20-1.65(m, 7 H); 1.86-2.36(m, 6 H); 2.60(dd, J=15,5 and 4, 1 H); 2.95(broad, 1 H); 5.39(broad s, 1 H); ˜10.3-11.3(broad, 1 H)δppm.

NMR(¹³C): 14.1(q); 22.6(t); 27.4(t); 28.9(t); 30.4(t); 30.5(t); 31.8(t); 38.5(t); 43.4(d); 124.6(d); 146.0(s); 180.2(s)δppm.

MS: 196(M⁺, 31), 136(56), 125(10), 121(12), 107(20), 93(49), 91(41), 79(100), 77(62), 67(40), 53(21), 41(48), 29(39).

EXAMPLE 4

Preparation of (+)-m ethyl (1 R,2 S,3 R)-2,3 -epoxy-2-pentyl-1-cyclopentaneacetate

A solution of the ester described in Example 3 (2.10 g, 10.0 mmole) and maleic anhydride (1.46 g, 15.0 mmole, 1.5 eq.) in CH₂Cl₂ (15 ml) was treated dropwise, at 10°, with 70% H₂O₂ (0.58 g, 12 mmole, 1.2 eq.). After 4 h at 10°, the reaction mixture was poured into an aqueous saturated solution of NaHCO₃. The product was extracted with ether, washed with water and with brine, dried over Na₂SO₄ and evaporated to give 2.35 g of crude product. Distillation in a bulb-to-bulb-apparatus (oven temp. 70°/8 Pa) gave the epoxide in a pure state (1.95 g, 86%), with a diastereomeric excess d.e. of 100% and an enantiomeric purity of 90% e.e. (column Megadexo® 5).

An identical result was obtained when 70% H₂O₂ (360 mg, 7.5 mmole) was used, added to a solution of (CF₃CO)₂O (2.31 g=1.53 ml, 11.0 mmole) in CH₂Cl₂ (20 ml) at 0°. After 30 min, this solution was added dropwise, at −50°, to a suspension of the ester described in Example 3 (1.05 g, 5.0 mmole) and of Na₂CO₃ (1.59 g, 15 mmole) in CH₂Cl₂ (10 ml). After the addition had been finished (15 min), the reaction was completed. The solution was treated as described above and the product distilled to give 863 mg (yield 76%) of pure (+)-methyl (1 R,2 S,3 R)-2,3-epoxy-2-pentyl-1-cyclopentaneacetate, showing characteristics which were identical to those cited above.

[α]²⁰ _(D)=+13.3 (c=3.7; CHCl₃)

NMR(¹H, 360 MHz): 0.89(t, J=7, 3 H); 1.11(m, 1 H); 1.22-1.40(m, 6 H); 1.48-1.66(m, 2 H); 1.76-1.91(m, 2 H); 1.95(dd, J=14 and 8, 1 H); 2.22-2.38(m, 2 H); 2.60(dd, 15,5 and 4, 1 H) ; 3.29 (s, 1 H); 3.66(s, 3 H)δppm.

NMR(¹³C): 14.0(q); 22.6(t); 24.7(t); 26.5(t); 26.7(t); 29.2(t); 31.9(t); 34.5(t); 37.6(d); 51.6(q); 62.6(d); 68.3(s); 173.6(s)δppm.

MS: 226(M⁺, 3), 211(12), 183(26), 167(35), 152(42), 138(22), 123(37), 109(51), 96(89), 81(100), 67(56), 55(72), 41(88), 29(49).

Moreover, other reaction conditions were also used. The table hereinafter summarizes and compares the above-described results to those obtained by varying the epoxidation conditions. The various compounds (absolute configurations) present in the reaction product (weight %), as well as their retention time on a column of the type DB-WAX [100° (3 min), then 30°/min], are indicated in the table.

TABLE I Products (%)

Run Reaction conditions (5′37) (6′59) (6′68) (10′24) (13′02) 1 (CF₃CO)₂O/H₂O₂/ Na₂CO₃/CH₂Cl₂/−50° 10 min — 97 — 1 — extr. 91 2 1 2 maleic anhydride/ H₂O₂/CH₂Cl₂/10° 2 h 32 58 1 — 6 4 h; extr. — 84 — 1 10  3 maleic anhydride/ H₂O₂/toluene/10 → 30° 1 h  3 77 6 — 5 4 h extr.  1 80 1 — 9 4 HCOOH/HCOONa/ H₂O₂/40° 6 h 72 21 4 not det. 5 m-chloroperbenzoic acid/CH₂Cl₂/0° 15 min — 83 14  — — extr. 6 CH₃CO₃H/CH₂Cl₂/20° 1 h  4 80 9 — — 2 h  1 78 12  — — extr. — 83 12  — —

EXAMPLE 5

Preparation of (+)-ethyl (1 R,2 S,3 R)-2-epoxy-2-pentyl-1-cyclopentaneacetate

This epoxide was prepared under conditions similar to those described in Example 4, but starting from the ester described in Example 2. The table below summarizes the results obtained under the different reaction conditions.

TABLE II Products (%) Run Reaction conditions

1 (CF₃CO)₂O/H₂O₂/ Na₂CO₃/CH₂Cl₂/−50° 10 min — 91 — — — 2 maleic anhydride/ H₂O₂/CH₂Cl₂/10° 2 h — 82 — 1 11 3 m-chloroperbenzoic acid/CH₂Cl₂/0° 15 min — 77 13 — — extr. 4 CH₃CO₃H/CH₂Cl₂/20° 1 h 3 83 12 — — 2 h 1 84 — — 11

EXAMPLE 6

Preparation of (+)-methyl (1 R)-cis-3-oxo-2-pentyl-1-cyclopentaneacetate or (+)-(1 R)-cis-Hedione®

A mixture of the epoxide described in Example 4 (1.0 g, 4.42 mmole), 4 Å molecular sieve (0.50 g) and toluene (10 ml) was treated at 23° with trifluoroboron etherate BF₃.OEt₂ (56 μl=63 mg, 0.44 mmole, 0.1 eq.). After 8 min at 20-25°, the reaction was stopped by adding a saturated, aqueous, icy solution of sodium bicarbonate, then the product was extracted with ether, washed two times with water and one time with brine, dried over Na₂SO₄ and the solvent evaporated to give 1.09 g of crude product. Distillation of the latter in a bulb-to-bulb apparatus (oven temp. 60-85°/7×10² Pa) gave 824 g of (+)-(1 R)-cis-Hedione® with a purity of 80% (yield 66%, 87% d.e., 90% e.e.). A second fractional distillation gave the (+)-(1 R)-cis-Hedione® containing 5 weight % of the isomer of (−)-(1 S)-trans configuration, as well as 5 weight % of a diene, i.e. methyl(2-pentyl-2-cyclopenten-1-ylidene) acetate.

When, in the same procedure, stoichiometric amounts of BF₃.OEt₂ were used, the (+)-(1 R)-cis-Hedione® was obtained with 90% purity (90% d.e., 90% e.e.), in 70% yield.

Moreover, other reaction conditions, distinct from those previously mentioned were used in analogous manner. The table below summarizes and compares the above-mentioned results to those obtained under different reaction conditions. The different compounds (absolute configurations) present in the reaction product (weight %), as well as their retention time on a column of the type DB-WAX (100° (3 min), then 30°/min) are indicated in the table.

TABLE III Compounds (% by weight)

Starting Run Reaction conditions (7′48) (7′30) (6′70) (10′24) epoxide 1 AlCl₃ (0.1 eq.), CH₂Cl₂, 5° 10 min 86 6-7  5  2 — 2 and 3 BF₃.OEt₂ (1 eq.), tol., 20° 10 min 88 4  8 — — a) 4 Å molecular sieve (50%) b) BF₃.OEt₂ (0.1 eq.), tol., 20° 5 min 82 6  9  2 — 4 MgI₂ (1 eq.), tol., 110° 10 min 60 2 16 — — 5 Filtrol ® G24 (10%), tol., 110° 30 min 56 5  9 20 — 6 Sc(OSO₂CF₃)₃ (0.5 eq.), CH₂Cl₂, 20° 15 min 78 6  7  9  0 7 MgBr₂, tol., 110° 3 h.  1 — —  1 88 8 LiClO₄, Et₂O, 5° 24 h 24 — 23 23 28 9 LiClO₄, tol., 24° 63 h 54 1 23  5  5 10  MgClO₄, tol., 50° 1 h 31 — 34 — 35

EXAMPLE 7

Preparation of (S)-tetrahydro-1-methyl-3-phenyl-1 H,3 H-pyrrolo[1,2-c][1,3,2] oxazaborol

A solution of (1 R,2′S)-1-phenyl-2′-pyrrolidinemethanol (obtained according to Ookawa et al., ref. cited) (0.554 g, 3.1 mmol) and trimethylboroxine (0.226 g, 2.1 mmol) in toluene (10 ml) was stirred at 25° C. After 45 min, the solution was slowly warmed up to reflux during 2 h and H₂O and the boronic acid formed were distilled off as an azeotropic mixture with toluene. Once the mixture was concentrated by half, fresh toluene (ca. 5 ml) was added (3 or 4 volumes of toluene needed to be added during the reaction). The reaction mixture was allowed to cool to room temperature overnight and the solution of the desired oxazaborolidine (5.2 ml, ca. [0.59 M]) was transferred via syringe into a bottle and stored under argon. The solution could be used as such in the reduction reactions described above.

MS: 201(M⁺, 100), 200(94), 186(12), 172(92), 158(20), 130(42), 104(19), 91(78), 77(18), 70(18), 67(18), 51(13), 39(15).

COMPARATIVE EXAMPLE

Process described in WO 95/33735

A. Methyl 2-pentyl-2-cyclopentene-1-acetate was epoxidized under the conditions described in the above-cited document, Example 1 (Na₂SO₄, HCOOH, HCOONa, H₂O₂ at 70%, 40°, 6 h). After ether extraction and distillation in a bulb-to-bulb apparatus of the reaction product, there was obtained essentially the starting ester, as is apparent from table I presented above (see run 4), the yield in the desired epoxide being only ˜20%.

B. c-Methyl 2,3-epoxy-2-pentyl-r-1-cyclopentaneacetate (obtained in an analogous way to that described in Example 4, but starting from racemic methyl 2-pentyl-2-cyclopentene-1-acetate) was reacted in a way analogous to that described in example 2 of WO 95/33735.

A mixture of 400 mg of the above-mentioned epoxide (1.80 mmole) and 19 mg of LiI (0.14 mmole) was placed in a bulb-to-bulb apparatus and heated to 190°. After 10 min, the black mixture was distilled in vacuo (8 Pa). The distillate (277 mg) consisted essentially of the starting epoxide (82% pure). The distillate was again combined with the residue (68 mg) and heated 30 min to 190°, cooled and distilled in a bulb-to-bulb apparatus (oven temp. 90°/8 Pa) to obtain 110 mg of distillate. Redistillation of this product (oven temp. 110°/11 Pa) gave 85 mg of a product having a purity of 90% and showing the following analytical data.

IR (neat): 2950, 2920, 2855, 1760, ˜1735(shoulder), ˜1705(shoulder), 1450, 1215, 1185 cm⁻¹.

NMR(¹H, 360 MHz): 0.87(t, J=7, 3 H); ˜1.2-1.8(m, 12 H); 1.90(m, 1 H); 2.05(m, 1 H) ; 2.28(dd, J=17,5 and 3,5, 1 H) ; 2.51(m, 1 H) ; 2.85(dd, J=17,5 and 10, 1 H)δppm.

NMR(¹³C): 177.5(s); 98.3(s); 42.1(d); 39.4(t); 38.1(t); 37.1(t); 34.4(t); 32.1(t); 24.0(t); 23.9(t); 22.5(t); 14.0(q)δppm.

MS: 196(M⁺, 6), 167(6), 153(10), 140(17), 125(100), 97(70), 81(18), 71(26), 69(30), 55(50), 41(69).

These spectral data correspond to the lactone the structure of which was cited further above in the text. 

What is claimed is:
 1. A method for the preparation of a cyclic ketone of formula

having a cyclanic configuration (1 R)-cis and in which R represents a lower alkyl group and R¹ represents a saturated or unsaturated, linear or branched hydrocarbon group having from 1 to 8 carbon atoms, wherein an epoxide of formula

having a cyclanic (1 R) configuration, the group in position 2 being in a trans configuration and the epoxy group being in a cis configuration with respect to that in position 1 of the ring, and in which R represents a lower alkyl group and R¹ represents a saturated or unsaturated, linear or branched hydrocarbon group having from 1 to 8 carbon atoms, is treated, in an inert organic solvent, with an acidic agent consisting of an appropriate Lewis acid or an acidic clay.
 2. The method according to claim 1, wherein (+)-methyl (1 R,2 S,3 R)-2,3-epoxy-2-pentyl-1-cyclopentaneacetate is used to prepare (+)-methyl (1 R)-cis-3-oxo-2-pentyl-1-cyclopentaneacetate.
 3. The method according to claim 1, wherein the acidic agent is trifluoroboron etherate or aluminium trichloride. 